Prudent Parasite Prevention

Did you read about recent worries pertaining to a popular flea/heartworm/intestinal parasite prevention product called Trifexis? Some folks in the Atlanta area felt certain that this product caused the deaths of their dogs. Although autopsy examinations did not implicate Trifexis as the cause, such stories can create significant stress and anxiety for people, particularly for those already experiencing stress and anxiety about giving so many medications to their dogs for parasite control. Is all this recommended prevention truly necessary?

Below, I have provided you with my feelings about parasite prevention. Please keep in mind that the following thoughts represent simply one veterinarian’s opinion. This information should not replace discussion and decision-making with your own vet. Your choices should be based on your pet’s potential exposure to various parasites as well as your own personal preferences.

Heartworm prevention

It used to be that I could tell you that if your dog lived in a particular state, city, or community you could skip heartworm prevention for your pets. Not so anymore. Heartworm disease now exists in every state within the United States. While it is intuitive to believe that a thick, long coat or exclusively indoor lifestyle will protect an animal from heartworm disease, this is not the case. Wherever an animal lives and whatever the plushness of their hair coat, year-round heartworm prevention is a good idea.

Here’s the good news about heartworm preventive medications. With rare exception, they are exceptionally safe. Adverse side effects are few and far between. Now, the bad news- heartworm resistance to commonly used preventive drugs has become a real phenomenon, documented most clearly in dogs living in the Mississippi Delta.  The American Heartworm Society is currently funding research to learn more about heartworm resistance. For this reason, it is important to have dogs tested annually for heartworm disease even when heartworm preventive is being conscientiously administered.

Heartworms can cause serious damage and treatment of this disease is no walk in the park. I strongly recommend year-round heartworm preventive medication for your dogs and cats. The main exception to this recommendation applies to very elderly or infirmed pets who are not expected to live more than six months or so. This is the approximate amount of time it takes for significant symptoms to arise following heartworm infection.

FYI- my pets receive heartworm prevention once a month year round. I reside in the mountains of western North Carolina where the temperature drops below freezing during the winter.

Prevention of intestinal parasites (worms)

Dogs and cats are most likely to have intestinal parasites when they are puppies and kittens. This is because dormant stages of some intestinal worms are activated in the pregnant dog or cat and then “shared” with their offspring. Geography and lifestyle tend to dictate whether adult animals harbor intestinal parasites. For example, a dog living in North Carolina who frequents a busy dog park is far more likely to have intestinal worms than a Wyoming dog who never sets paw where other dogs tread.

Talk with your veterinarian to determine if intestinal parasite prevention is truly needed. Simply checking a stool sample for parasite eggs (this test is referred to as a fecal flotation) once or twice a year may be the way to go. Such testing is inexpensive and efficient. Worst-case scenario and your pet develops intestinal parasites- know that the treatment for this issue is safe, effective, and relatively inexpensive. If you are looking to thin out your pet’s menu of preventive medications, this would be a good one to consider dropping.

FYI- my pets do not receive intestinal parasite prevention. Rather, I run a fecal flotation once or twice a year to assess whether or not worms are present.

Flea prevention

Flea prevention falls on the “optional” list in terms of parasite prevention. Some environments simply don’t propagate much in the way of fleas. For example, a strictly indoor kitty may remain flea-free. Fleas don’t thrive at high altitude, so flea prevention medications would be unnecessary in mile-high Denver, Colorado. Additionally, for some pets and their humans, living with a few fleas simply isn’t much of a nuisance. For your situation, use of flea prevention medication only during the “flea season” may work well. Or, it may be that year-round administration is required. Here’s the bottom line. Assess your pet’s environment and tolerance for fleas. A few flea bites may be the lesser of two evils when contemplating fleas versus flea prevention.

FYI- my pets receive monthly flea prevention during our “flea season”. They do not receive flea prevention during the winter months.

Tick prevention

The incidence of tick-borne diseases is on the rise throughout much of the United States. Of all of these diseases, Lyme seems to get the most attention, perhaps because this disease is so common in people. Other significant tick-borne diseases include Rocky Mountain Spotted Fever, Ehrlichia, Anaplasma, and Bartonella. The incidence of these diseases varies widely from region to region within the United States. These infectious organisms can cause significant symptoms that may be life threatening. Additionally, in some cases they can be difficult to diagnose and/or treat effectively. The way to prevent these diseases is via effective tick prevention. This can take the form of avoidance of tick habitats during the tick season, “tick collars”, preventive medications, and/or careful tick surveillance (literally going over your pet with a fine tooth comb at least once a day).

To determine the best course of tick prevention in your household I recommend the following two things. First, determine if your pet has tick exposure. Have you ever seen one on your pet? Secondly, talk with your veterinarian about the incidence of tick-borne diseases in your community.

FYI- my pets receive monthly tick prevention medication, but only during the “tick season”.


Here are some important take home messages to help you create a prudent parasite prevention program for your pets:

  • Year-round heartworm preventive medication is important regardless of where you live.
  • Not all pets require year-round medications to prevent ticks, fleas, and intestinal parasites. For some animals, they may be important only during certain times of the year. For others, they are not needed at all.
  • Double check that the prevention product you are using contains only the medications your pet truly needs. Avoid giving unnecessary medications simply because this happens to be the product on your veterinarian’s pharmacy shelf.
  • Any parasite prevention medication is capable of producing an adverse or idiosyncratic response in any particular animal. If your pet develops symptoms on the heels of administrating such a product, contact your family veterinarian right away.

What parasite prevention products do you use?

Best wishes,

Nancy Kay, DVM

Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
Become a Fan of Speaking for Spot on Facebook

Please visit to read excerpts from Speaking for Spot and Your Dog’s Best Health.   There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at,, local bookstores, and your favorite online book seller.


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20 Comments on “Prudent Parasite Prevention

  1. Dr. Kay, you wrote: “While it is tempting to use the large animal preparation of ivermectin, veterinarians (including myself) typically discourage this because it is so darned easy to make dosing errors, resulting in ivermectin toxicity. If use of the large animal product is imperative because of financial constraints, I encourage folks to work with their family veterinarians or a staff technician to review appropriate dosing. ” And I certainly agree. It helps that I have the best vet in the world; sadly, many do not :-( But I always advocate finding a great vet and working with them; mine is a huge part of my life, because I have so many pets and I do rescue, so I am there just about every week. Dosing IS important: while an extra drop of bovine injectable in one of my 60+ pound dogs may not be a problem, with a small breed it could be. If I could afford it, I’d rather just give Heartguard: it is so much simpler. BUt the bovine injectable is a great solution for those who really can’t afford the others, and a lifesaver for HW+ dogs needing rescue. Of course, everything should be done in conjunction with a vet! Although I am able to administer shots and subcutaneous injections, do microchipping and other vet care, I always involve my vet. Thanks for all the info you share with us!

  2. Hi Lyn. Thanks very much for drawing this to my attention. I was unaware of the contraindication of Spinosad with seizures. I searched on Veterinary Information Network where this was discussed and the risk very much downplayed. Nonetheless, a warning is a warning! I might also mention here that, any drug in any given individual is capable of inducing an idiosyncratic response. Is it possible that a parasite prevention product could cause seizures in a normal dog? Anything is possible when it comes to medication side effects.

  3. Spinosad, the active ingredient in Comfortis and Trifexis, has a warning on the label to use with caution with animals with pre-existing seizures. Seizures are listed among adverse reactions. The cases in Atlanta and many others may or may not have been caused by spinosad. Like other insecticides in the past, there is no telling if problems with thos product or others will be confirmed in the future.

    I agree that in most cases year round use of heartworm preventatives is a good idea. However, which one are you using that does not include a flea insecticide? I would hope that we would be given back the choice of Interceptor without any flea killer.

  4. Hi Elizabeth. While it is tempting to use the large animal preparation of ivermectin, veterinarians (including myself) typically discourage this because it is so darned easy to make dosing errors, resulting in ivermectin toxicity. If use of the large animal product is imperative because of financial constraints, I encourage folks to work with their family veterinarians or a staff technician to review appropriate dosing. Hope this helps.

  5. Hi again Dr. Kay-
    I have a multi-dog household. I feel it is necessary to keep my dogs on heartworm preventative but find that the cost makes it difficult to use the usual medications. I use Ivermec (for cattle) and have done so for many years. None of my dogs have ever tested heartworm positive.
    I’d like to hear your opinion of this. I might add that none of my dogs have one iota of collie in them.

  6. A very good thread, Dr. Kay. As always :)

    Some questions about heartwormers…

    If preventative resistant parasite strains are emerging, wouldn’t it make more sense administer preventative only during warm weather when the vectors are active to reduce exposure and, so, reduce the development of resistance effects?

    Also, given the resistance issue, is Interceptor likely to make a re-appearance? A while back, Pfizer said they were not going to make it any more but there is a market for it, and if ivermectin becomes impotent, that’s another opportunity for Pfizer’s business.

    Speaking of the clamor for Interceptor’s return… Heartworm preventatives are not benign. Ivermectin and any of the -mectins can be dangerous for dogs with MDR1 mutation. HeartGard is safe at the dosing level recommended, according to Washington State’s vet college. However, MDR1 testing is a good idea especially for breeds predisposed to that mutation. Knowing a dog’s MDR1 status helps owners work together with their vet to provide the best care for individual animals.

    There are plenty of other drugs on the MDR1 toxic list, including Loperamide… So, people should be very careful about following those dopey Facebook posts that tell you to build an emergency first aid kit with some tape, a piece of string, and $1.50 at Walmart in lieu of consulting an expensive vet… People can kill their dog or cat following that bad advice.

    I have to admit, like other posters on this thread, I am skeptical about year-round treatment. I had someone explain why year round preventatives were of value from the perspective of microfilaria-host interaction but I don’t remember it… ??

  7. Hi Jane. Please read my response to Elizabeth regarding use of the large animal formulation of ivermectin.

  8. This is a fine can of worms, pun intended! I have been using advantage for fleas, but it seems to be losing its efficacy; so I need to find something to use that is safe to have around cats (I know some are not). I treat my carpets with boric acid to kill the larvae, and I do not treat my yard. I only use flea meds when I see the dogs scratching. When I only had 2 huskies, when they started scratching, I would simply give them a flea bath. I dislike all the flea meds, but due to a back injury and low budget, I now have to use them sometimes.
    My dogs rarely get worms; they are on ivermectin HW preventative year round, which also kills some worms. I keep an eye out for tapeworm eggs which I find every few years, and of course, I try to watch them defecate periodically, since I have several dogs, and otherwise I don’t know what came out of who!
    In tick season we avoid tick habitat as much as possible. I examine the dogs for ticks daily during the season. Having huskies, I often find ticks swimming around in their fur muttering “I know there’s a dog in here somewhere!” and they DO get in.
    I have been through HW treatment with several dogs; it is horrendous, and something I don’t ever want to happen to any dog. I used to worry about giving it all the time, until I learned that the treatment for demodex mange is massive doses of ivermectin…so a few extra drops will not hurt. I also give it to my cats: as my vet pointed out, mosquitos DO get into the house. I hope this may be a good place to share some info on HW:
    When I was with a dog rescue, several of us had a long visit with one member’s personal vet who has successfully treated several of our dogs, and I don’t know how many dozen others. This costs a fraction of traditional heartworm treatment. I hope this will be shared with as many rescue organizations as possible; I know that before this, we sometimes had to pass on saving a dog with heartworms because we just couldn’t afford the treatment ($500-$800 for the HW+ dogs I personally have fostered) A bottle of bovine injectable Ivermectin cost me $50 and would see a dog through the entire treatment, I believe. To dose my dogs monthly, I put 1/3cc of Ivermectin on a piece of bread and when it has soaked in, I put a smidge of butter on it, so it is eaten immediately.
    The idea here is that heartworms have a symbiotic relationship with some bacteria. By beginning treatment with doxycycline, the bacteria is killed, which weakens the heartworms. Then you hit them with the Ivermectin, daily. This way the HWs die in small amounts; the dog does not have to be kept quiet! As more HWs die, the dose is increased, killing the stronger HWs, until they are all dead and slowly pass out of the dogs system.
    I had considerable trouble getting the propylene glycol; I finally ordered it through the local CVS pharmacy; others have gotten it at farm supply places.

    no painful shots of poison for the dogs
    no restricted activity
    rescues don’t have to let dogs die because they can’t afford the HW treatment!

    Negative: the person in care of the dog must be not only willing, but capable of not missing doses!

    Dog gets dosed EVERY DAY for at least 6 months. After 6 months, re-test for HW; if still positive, continue to administer Ivermectin at highest dose for another 3 months; then re-test. Keep re-testing until the heartworms are gone. This usually takes 6 months; but may on occasion take longer.

    0.82 cc of Ivermectin with 1 oz. propylene glycol (total 30cc) = 272mg/cc

    Monthly dose:
    1cc = dose for 51-100lbs.
    ½ cc = dose for 26-50 lbs.
    ¼ cc = dose for up to 25 lbs

    1cc concentrate in 9cc propylene glycol =1,000mg/cc

    Treatment: for 60 pound dog (average weight of huskies; adjust dosages as necessary)
    1st 2 weeks: give 3 tablets doxycycline (300mg) once a day (42 tablets)

    Then give 200 micrograms Ivermectin once a day for one month (.02 cc concentrate) or .2cc at 1:10 dilution

    Then give 400 micrograms Ivermectin daily for one month.

    Then give 800 micrograms once a day until cleared; may take 6 months total

    After 6 months, retest.

    The only dogs of concern in ivermectin sensitivity are collie breeds/herding breeds. There is testing but it would be needed on a case by case basis when herding breed is suspected.

  9. Hi Irma,
    Great question. There are no parasite preventive medications that will promote seizure activity in dogs with or without epilepsy. Also, none of these preventives are incompatible with medications that might be used to prevent seizures. In other words, you can use whichever product your veterinarian recommends.

  10. Hi Dr. Nancy — We spend from about May to mid-October in the Pacific Northwest (WA) and late October to April in the Phoenix, AZ area. We give our 10 year old Tibetan Terrier Heartguard heartworm meds all year round. We start her on Advantix II prior to returning to Washington and keep her on it until we return to AZ. Have not had any problems with this routine, either for the dog or with reactions, during the last 6-8 years we have been doing it. Will continue to protect her with these meds, since they certainly seem to work for us!

  11. I agree with Linda. I think our dogs receive far too many noxious ingredients in their systems as it is. I try to use common sense, natural means of prevention and treatment and appropriate feeding.

    I remember when heartworm suddenly appeared at the vet’s office. After the first year I enquired how many cases they had seen. None.

  12. I live in Winnipeg, Manitoba and we only see mosquitoes for 5 months of the year. I was actually considering stopping the heartworm meds and testing at the beginning, middle and end of the season. Would that work? My reason to stop is to lessen the exposure to medications.

  13. We live on the border of North Central Florida and South Georgia. Swampy farming community with the hardiest fleas ever seen. We use TRIHEART Plus year-round and Comfortis almost year-round. When ticks are seen, we switch to ADVANTIX II. My husband and I have run a spay/neuter clinic for six years and seen well over 13,000 dogs and cats within a 70-mile radius, so we have a good handle on the situation. These are the products that work here and we recommend/sell to clients.

  14. Hi Linda,
    You have posed a great question. You and several others are wondering if it is reasonable to skip giving heartworm preventive medication when temperatures are consistently at or below freezing. There are a few reasons why the American Heartworm Society advises against this. One issue is compliance- some people forget to resume giving heartworm prevention when warmer weather rolls around. Secondly, local climatic factors can vary dramatically from year to year in any particular locale. Additionally, many pets travel with their humans, particularly around the winter holidays. Lastly, consistent monthly doses of heartworm preventive have been shown to more effective against challenging/resistant strains of heartworm. Oh yes, and one more thing. The monthly preventives protect against intestinal parasites that can be transmitted year round. All this being said, could I fault those of you living in regions such as Minnesota Maine for skipping your pet’s heartworm preventive November through February? I would have a hard time getting hot and bothered about this.

  15. In places where it freezes in the winter there are no mosquitoes for 6 months or so. Since mosquitoes are necessary to transmit heartworm disease, it makes no sense to keep giving HW poisons. I think the pharmaceutical companies are advocating year-round compliance because their products are becoming less effective than they used to be!

  16. I used Trifexis for a very short time and then returned the balance of the last package to my vet. It smelled funny to me and my dogs acted like I was trying to poison them although they have always been very eager to take any other palatable and chewable medicine I give them.

  17. I should have said my Addison dog is being treated for his Pulmonary Hypertension with Sildenafil. It is working wonderfully.

  18. Hi Peggy. Please double check with your family veterinarian to be sure that heartworm disease has not made its way to Vancouver Island. From a geographical point of view, this disease is spreading rapidly. Also, if you travel with your Papillon, you might want to begin heartworm prevention. In terms of what product to use to prevent fleas and ticks, I encourage conversation with your family vet. He or she should have a feel for which product(s) is working best in your neck of the woods. I hope this helps.By the way, lucky you for having a tiny dog with Addison’s disease. Most dogs who get this disease are giant breeds and, as you know, the costly treatment is dosed based on body weight!

  19. Hello Dr. Kay:
    I have a 13 year old Papillon who has lived well with Addison’s Disease since he was 7. His Addison’s is well controlled on Percorten and Prednisone. He also has Pulmonary Hypertension. I also have a 3 year old healthy Papillon. We currently live on Vancouver Island in BC, Canada. But may need to move off Island. Currently, there is no heartworm on Vancouver Island but it is everywhere else in Canada. What flea, tick and heartworm medication would you use in my case. Thank you in advance for your time.